This is a patent application based on a Japanese patent application No. 2001-171870 which was filed on Jun. 7, 2001 and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a Group III nitride compound semiconductor light-emitting element containing a plurality of low-energy-band-gap layers which emit light of different wavelengths.
2. Background Art
In order to attain mass production of, for example, white LEDs, many studies have heretofore focused on semiconductor light-emitting elements containing a plurality of low-energy-band-gap layers which emit light of different wavelengths.
FIG. 13 shows energy band diagrams of conventional semiconductor light-emitting elements, each containing two low-energy-band-gap layers which emit light of different wavelengths.
FIG. 13A shows an energy band diagram of a semiconductor light-emitting element containing two low-energy-band-gap layers which emit light of wavelengths that are arbitrarily or appropriately different, and containing no quantum-barrier-formation barrier layer between the low-energy-band-gap layers. FIG. 13B shows an energy band diagram of a semiconductor light-emitting element containing two low-energy-band-gap layers which emit light of wavelengths that are arbitrarily or appropriately different, and containing a simple quantum-barrier-formation barrier layer between the low-energy-band-gap layers.
As shown in FIG. 13A, when no quantum-barrier-formation barrier layer is provided between the two low-energy-band-gap layers, electrons and holes tend to migrate to the layer having lower band gap energy; i.e., the layer which emits light of long wavelength. Therefore, difficulty is encountered in xe2x80x9cobtaining a desired ratio of emission intensity between the low-energy-band-gap layersxe2x80x9d; for example, causing the two low-energy-band-gap layers to emit at substantially the same intensity. Therefore, a desired light color is difficult to obtain by mixing emitted light of different colors.
As shown in FIG. 13B, when a simple barrier layer is provided between the two low-energy-band-gap layers, electrons are easily distributed to one of the low-energy-band-gap layers and holes are easily distributed to the other. Therefore, even when a desired emission intensity ratio is obtained, emission efficiency is lowered at the two low-energy-band-gap layers. That is, the resultant semiconductor element fails to exhibit sufficient emission intensity and emission efficiency.
As shown in FIGS. 13A and 13B, when overflow of carriers is effectively prevented by increasing energy barriers of p-type and n-type semiconductor layers provided on the two low-energy-band-gap layers, the p-type and n-type semiconductor layers must be increased in thickness, and the semiconductor layers must also be increased in aluminum (Al) content. As a result, the semiconductor element is prone to cracking.
In view of the foregoing, an object of the present invention is to provide a semiconductor light-emitting element containing a plurality of low-energy-band-gap layers which emit light of different wavelengths, which easily attains xe2x80x9cdesired proportions in emission intensity between the low-energy-band-gap layersxe2x80x9d and which exhibits excellent emission efficiency and durability.
In order to achieve the above object, the present invention employs the following means.
According to first means of the present invention, there is provided a Group III nitride compound semiconductor light-emitting element formed of Group III nitride compound semiconductor layers, comprising multiple layers containing light-emitting layers (hereinafter collectively called xe2x80x9ca multi-layer containing light-emitting layersxe2x80x9d or xe2x80x9ca multi-layerxe2x80x9d), a p-type semiconductor layer, and an n-type semiconductor layer, wherein the multi-layer comprises a multiple quantum barrier-well layer containing quantum-barrier-formation barrier layers formed from a Group III nitride compound semiconductor and quantum-barrier-formation well layers formed from a Group III nitride compound semiconductor, the barrier layers and the well layers being laminated alternately and cyclically, and a plurality of low-energy-band-gap layers which emit light of different wavelengths; and the multiple quantum barrier-well layer is provided between the low-energy-band-gap layers. Here xe2x80x9clow-energy band gap layerxe2x80x9d also includes a well layer.
As used herein, the expression xe2x80x9cGroup III nitride compound semiconductorxe2x80x9d encompasses binary, ternary, and quaternary semiconductors of arbitrary compositional proportions represented by the following formula: AlxGayIn(1xe2x88x92xxe2x88x92y)N (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61); and semiconductors containing trace amounts or small amounts of p-type or n-type impurities, the impurities having substantially no effect on the compositional proportions x and y.
Therefore, the expression xe2x80x9cGroup III nitride compound semiconductorxe2x80x9d encompasses binary and ternary Group III nitride compound semiconductors, such as AlN, GaN, InN, AlGaN of arbitrary or appropriate compositional proportions, AlInN of arbitrary or appropriate compositional proportions, and GaInN of arbitrary or appropriate compositional proportions; and semiconductors containing trace amounts or small amounts of p-type or n-type impurities, the impurities having substantially no effect on the compositional proportions.
The expression xe2x80x9cGroup III nitride compound semiconductorxe2x80x9d encompasses semiconductors in which the aforementioned Group III element (Al, Ga, or In) is partially substituted by boron (B) or thallium (Tl), or in which nitrogen (N) is partially substituted by phosphorus (P), arsenic (As), antimony (Sb), or bismuth (Bi).
Examples of the aforementioned p-type impurity include magnesium (Mg) and calcium (Ca).
Examples of the aforementioned n-type impurity include silicon (Si), sulfur (S), selenium (Se), tellurium (Te), and germanium (Ge).
These impurities may be incorporated in combination of two or more species, and a p-type impurity and an n-type impurity may be incorporated in combination.
The aforementioned low-energy-band-gap layer may have an SQW structure or an MQW structure.
FIG. 1 illustrates the function of a multiple quantum barrier-well layer employed in the present invention. FIG. 1A shows an energy band diagram of the multiple quantum barrier-well layer. In FIG. 1A, reference letter Wb represents the thickness of a quantum-barrier-formation barrier layer constituting the multiple quantum barrier-well layer, Ww represents the thickness of a quantum-barrier-formation well layer constituting the multiple quantum barrier-well layer, and Vb represents the energy level of the quantum-barrier-formation barrier layer.
When electrons are applied to the multiple quantum barrier-well layer from the left side of the drawing as shown in FIG. 1A, the probability that the electrons are reflected to the left side (i.e., reflectance) is classically thought to be as shown in FIG. 1B. In FIGS. 1B and 1C, reference letter E on the x-axis represents the kinetic energy of electrons applied in a forward direction.
However, in reality, electrons behave on the basis of the quantum theory. Therefore, by virtue of the tunnel effect or interference of electronic waves, the reflectance of electrons to the multiple quantum barrier-well layer substantially corresponds to the results of a quantum-theoretical simulation shown in FIG. 1C. In FIG. 1C, me (=0.2) represents the ratio of the effective mass of conduction electrons to the rest mass of electrons in the multiple quantum barrier-well layer, the ratio being employed in the simulation.
When xe2x80x9cdxe2x80x9d represents the length of xc2xd the lamination cycle of each of the quantum-barrier-formation barrier layers constituting the multiple quantum barrier-well layer (2.5 nm in the case shown in FIG. 1) and xe2x80x9cxcexxe2x80x9d represents the wavelength of conduction electrons injected to the multiple quantum barrier-well layer, conduction electrons satisfying the following relations exhibit high reflectance: d=xcex/4, d=3xcex/4.
In contrast, conduction electrons satisfying the following relation exhibit low reflectance: d=xcex/2.
When the multiple quantum barrier-well layer exerting the aforementioned barrier effect is introduced, the height of an effective quantum barrier becomes considerably greater than that of a classical quantum barrier. In addition, conduction electrons having a specific kinetic energy E selectively pass through the multiple quantum barrier-well layer by virtue of the tunnel effect, even when the energy level of the conduction electrons is lower than the height of the classical quantum barrier. For example, as shown in FIG. 1C, conduction electrons satisfying the following relation: xcex=2d=5 nm (i.e., conduction electrons having a kinetic energy E of about 0.3 eV) exhibit low reflectance by virtue of the tunnel effect.
Therefore, in the case where the aforementioned multiple quantum barrier-well layer is employed, even when the amount of Al contained in p-type and n-type semiconductor layers is reduced, and these semiconductor layers are of reduced thickness as compared with conventional p-type and n-type semiconductor layers, overflow of carriers is effectively prevented, and optimal or suitable distribution of the carriers to low-energy-band-gap layers is realized. Thus, as compared with a conventional light-emitting element, the resultant semiconductor element emitting light of different wavelengths exhibits excellent emission efficiency and durability.
FIG. 2 is an energy band diagram illustrating the effect of the multiple quantum barrier-well layer of the semiconductor light-emitting element of the present invention. In FIG. 2, filled circles and open circles symbolically represent electrons and holes, respectively.
As shown in FIG. 2, when the multiple quantum barrier-well layer is provided between two low-energy-band-gap layers which emit light whose wavelengths are arbitrarily or appropriately different, distribution of electrons is determined on the basis of the quantum-theoretical effect as shown in FIG. 1. Distribution of holes is determined as shown in FIG. 2, and is similar to the distribution of electrons.
Therefore, when an appropriate multiple quantum barrier-well layer from which carriers having a specific kinetic energy E are reflected at a specific reflectance is employed in the first means of the present invention, the semiconductor light-emitting element containing a plurality of low-energy-band-gap layers which emit light of different wavelengths easily attains xe2x80x9cdesired proportions in carrier distribution (emission intensity) between the low-energy-band-gap layersxe2x80x9d and exhibits excellent emission efficiency and durability.
Since the aforementioned multiple quantum barrier-well layer exerts the effect of reducing stress applied to the low-energy-band-gap layers, the stress being attributed to strain in the light-emitting element, semiconductor layers of good crystallinity are grown, and the low-energy-band-gap layers, etc. maintain a high level of crystallinity. According to the first means, when the thicknesses, lamination cycles, compositional proportions, and number of semiconductor layers constituting the multiple quantum barrier-well layer are optically or suitably determined in accordance with the kinetic energy of carriers, the carriers can assume substantially uniform distribution between a plurality of the low-energy-band-gap layers, or desired proportions in carrier distribution between the low-energy-band-gap layers can be attained. More specifically, the following means are effective.
In second means of the present invention, preferably, the band gap of each of the quantum-barrier-formation barrier layers is determined to be larger than that corresponding to the wavelength of light emitted from each of the low-energy-band-gap layers.
In third means of the present invention, preferably, the thickness of each of the quantum-barrier-formation well layers or each of the quantum-barrier-formation barrier layers is determined to be 0.5 nm to 10 nm inclusive.
In fourth means of the present invention, preferably, each of the quantum-barrier-formation well layers is formed from AlxGayIn1xe2x88x92xxe2x88x92yN (0xe2x89xa6x less than 0.1, 0.8 less than y less than 1, 0 less than 1xe2x88x92xxe2x88x92y less than 0.1).
In fifth means of the present invention, preferably, each of the quantum-barrier-formation barrier layers is formed from AlxGa1xe2x88x92xN (0xe2x89xa6x less than 0.5).
In order to obtain light of a desired color from the semiconductor light-emitting element of the present invention, the aforementioned second through fifth means may be arbitrarily selected and effectively employed in combination.
For example, the multiple quantum barrier-well layer may have a structure in which the quantum-barrier-formation barrier layers, each having a band gap larger than that corresponding to the wavelength of light emitted from each of the low-energy-band-gap layers, are laminated cyclically. Alternatively, the multiple quantum barrier-well layer may have a structure in which each of the quantum-barrier-formation well layers is formed from AlxGayIn1xe2x88x92xxe2x88x92yN (0xe2x89xa6x less than 0.1, 0.8 less than y less than 1, 0 less than 1xe2x88x92xxe2x88x92y less than 0.1), each of the quantum-barrier-formation barrier layers is formed from AlxGa1xe2x88x92xN (0xe2x89xa6x less than 0.5), each of the quantum-barrier-formation well layers and the quantum-barrier-formation barrier layers is determined to have a thickness of 0.5 nm to 10 nm inclusive, and the quantum-barrier-formation well layers and the quantum-barrier-formation barrier layers are laminated cyclically. When the multiple quantum barrier-well layer having such a structure is employed, carriers can be assume a substantially uniform distribution between a plurality of the low-energy-band-gap layers which emit light of different wavelengths, or desired proportions in carrier distribution between the low-energy-band-gap layers can be attained. Therefore, according to the aforementioned means, light of a desired color can be obtained from the semiconductor light-emitting element.
FIG. 3 is a graph showing criteria for forming a suitable or optimal multiple quantum barrier-well layer. For example, when the quantum-barrier-formation well layers of the multiple quantum barrier-well layer are formed from AlxGayIn(1xe2x88x92xxe2x88x92y)N(0xe2x89xa6x less than 0.1, 0.8 less than yxe2x89xa61xe2x88x92xxe2x88x92y less than 0.1), and the quantum-barrier-formation barrier layers of the multiple quantum barrier-well layer are formed from AlxGa1xe2x88x92xN (0xe2x89xa6x less than 0.5), the transmittance of carriers (electrons and holes) becomes as shown in FIG. 3.
The relation shown in FIG. 3 is obtained under the following conditions.
(Condition 1) the ratio of the effective mass of conduction electrons to the rest mass of electrons in the multiple quantum barrier-well layer: me=0.2
(Condition 2) lamination cycle of the quantum-barrier-formation barrier layer: twice the thickness of the quantum-barrier-formation barrier layer (Wb=Ww)
(Condition 3) the number of quantum-barrier-formation barrier layers: 8
(Condition 4) operation voltage: 3 V
(Condition 5) temperature of the light-emitting element: about ambient temperature
(Condition 6) kinetic energy of conduction electrons in an incident direction: about 0.2 to 0.5 eV
As shown in FIG. 3, when the quantum-barrier-formation barrier layer has a thickness of 2.5 nm, and the compositional proportion of Al contained in the quantum-barrier-formation barrier layer is 0.3; i.e., x=0.3, the transmittance of carriers becomes about xc2xd. Therefore, carriers can assume substantially uniform distribution between the low-energy-band-gap layers provided on the respective surfaces of the multiple quantum barrier-well layer.
In the case where a plurality of multiple quantum barrier-well layers are provided between three or more low-energy-band-gap layers (each of the multiple quantum barrier-well layers is provided between the low-energy-band-gap layers), when the transmittance of carriers which pass through each of the multiple quantum barrier-well layers is optimally or suitably determined such that the carriers are distributed to the low-energy-band-gap layers in desired proportions, light of uniform intensity can be obtained from the low-energy-band-gap layers, or desired proportions in emission intensity between the low-energy-band-gap layers can be attained.
Such optimization may be carried out by means of, for example, calculation for obtaining proportions of carriers having a specific kinetic energy distributed to the low-energy-band-gap layers, on the basis of the transmittance of the carriers which pass through the respective multiple quantum barrier-well layers. Alternatively, such optimization may be carried out on the basis of the results of a variety of simulations and tests.
As described above, when the means of the present invention is employed, in the semiconductor light-emitting element containing a plurality of low-energy-band-gap layers which emit light of different wavelengths, quantitative optimization for improving emission efficiency and attaining desired proportions in emission intensity between the low-energy-band-gap layers; i.e., regulating color of emitted light, can be carried out easily.
Each of the aforementioned conditions 1 through 6 is not a necessary condition for the present invention. Therefore, there may be employed any multiple quantum barrier-well layer satisfying the following relation: Wb greater than Ww; i.e., a multiple quantum barrier-well layer containing a quantum-barrier-formation barrier layer having a thickness greater than that of a quantum-barrier-formation well layer.
That is, the aforementioned conditions 1 through 6 may be optimally or suitably varied in accordance with desired properties of the light-emitting element.
In sixth means of the present invention, preferably, operation voltage is varied to thereby change the color of emitted light.
As is clear from FIG. 1, the transmittance of electrons which pass through the multiple quantum barrier-well layer or the reflectance of electrons depends on, for example, the kinetic energy of the electrons. Furthermore, as shown by the aforementioned condition 4, the transmittance or reflectance depends on the operation voltage of the light-emitting element. Therefore, when the operation voltage of the light-emitting element is varied, carriers can be distributed to the low-energy-band-gap layers in arbitrary proportions. In addition, when the operation voltage of the light-emitting element is varied, proportions in emission intensity between the low-energy-band-gap layers can be regulated.
The color of light emitted from the light-emitting element is determined by colors of light emitted from the low-energy-band-gap layers; i.e., proportions of intensities of light emitted from the low-energy-band-gap layers. Therefore, according to the sixth means, the color of light emitted from the light-emitting element can be varied.
As described above, when carriers are distributed to the low-energy-band-gap layers in desired proportions, the semiconductor light-emitting element (i.e., single-chip-type element) can emit light of desired color by appropriately regulating operation voltage.
A conventional outdoor large-screen television employs three types of LEDs; i.e., a red LED, a green LED, and a blue LED. However, when the semiconductor light-emitting element (i.e., single-chip-type element) which emits light of desired color is employed, such a large-screen television may be produced from only one or two types of LEDs.
In seventh means of the present invention, preferably, there is provided a phosphor which receives light emitted from the low-energy-band-gap layer and emits light having a wavelength longer than that of the light emitted from the low-energy-band-gap layer.
When such a phosphor is provided, the amount of unwanted light of relatively short wavelength (e.g., UV light) which is emitted from the light-emitting element can be reduced, and the short-wavelength light can be converted to light of relatively long wavelength. Therefore, provision of the phosphor is effective particularly when light of a desired color is to be obtained through conversion of light by means of the phosphor.
Provision of the phosphor is also effective when a color of emitted light which has been converted by means of the phosphor is to be mixed with emitted light of other colors. According to the seventh means, a color of light emitted from the light-emitting element can be corrected.
In eighth means of the present invention, preferably, there is provided an impurity-added semiconductor layer which receives light emitted from the low-energy-band-gap layer and emits light having a wavelength longer than that of the light emitted from the low-energy-band-gap layer.
When such an impurity-added semiconductor layer is provided, the amount of unwanted light of relatively short wavelength (e.g., UV light) which is emitted from the light-emitting element can be reduced, and the short-wavelength light can be converted to light of relatively long wavelength. Therefore, provision of the impurity-added semiconductor layer is effective particularly when light of a desired color is to be obtained through conversion of light by means of the semiconductor layer.
Provision of the impurity-added semiconductor layer is also effective, when emitted light of a color which has been converted by means of the semiconductor layer is to be mixed with emitted light of other colors. According to the eighth means, a color of light emitted from the light-emitting element can be corrected.
The means employing an impurity-added semiconductor layer may be combined with the aforementioned means employing a phosphor. In some cases, this combination enables sufficient correction of a color of light emitted from the light-emitting element. However, in the case where light which has been converted by means of an impurity-added semiconductor layer has a high luminance, even when the eighth means is carried out singly, sufficient effect may be obtained.
In ninth means of the present invention, preferably, at least one element selected from among silicon (Si), sulfur (S), selenium (Se), tellurium (Te), and germanium (Ge), serving as an impurity, is added to the aforementioned impurity-added semiconductor layer. These impurities effectively convert light of relatively short wavelength, such as UV light, to visible light. Through addition of such an impurity, emission efficiency of the light-emitting element can be improved.
For example, when a semiconductor layer (e.g., a GaN layer) containing an appropriate amount of such an n-type impurity is irradiated with UV light, visible light is obtained through photoluminescence. Therefore, the ninth means is particularly effective when UV light is to be converted to visible light.
When an impurity-added semiconductor layer which receives UV light and emits yellow light is added to a semiconductor light-emitting element containing a low-energy-band-gap layer which emits UV light and a low-energy-band-gap layer which emits blue light, the resultant semiconductor light-emitting element emits white light.
In tenth means of the present invention, preferably, the aforementioned impurity-added semiconductor layer serves as an n-type semiconductor layer, an n-type contact layer, or a high-carrier-concentration n+ layer, and electrode formation or electrode connection is carried out through a flip chip process such that light is extracted through a substrate.
In the above case, emitted UV light passes through the aforementioned impurity-added semiconductor layer, and thus the amount of UV light which is converted to visible light increases as compared with the case of a wire-bonding-type light-emitting element.
Therefore, according to the tenth means, the color of emitted light can be corrected more effectively through conversion of the wavelength of the light.
When a low-energy-band-gap layer which emits red light is irradiated with photons of blue light, in some cases, electrons are excited in a light-emitting mechanism (band gap) of the low-energy-band-gap layer, and the excited electrons contribute to emission of red light in the mechanism. Through this effect, a portion of blue light is converted to red light. The greater the amount of light of short wavelength applied to a low-energy-band-gap layer which emits light of long wavelength, the greater the amount of light of short wavelength converted to light of long wavelength. That is, the greater the amount of light of short wavelength applied to the aforementioned impurity-added semiconductor layer, the greater the amount of light of short wavelength converted to light of long wavelength. The amount of light of short wavelength applied to a low-energy-band-gap layer which emits light of long wavelength greatly depends on the order of lamination of light-emitting semiconductor layers.
Since the order of lamination of light-emitting semiconductor layers is employed as a parameter for optimization of the color of light emitted from the light-emitting element or emission efficiency, when the lamination order is optimally or suitably regulated, the color of light emitted from the element can be easily converted to white.
In eleventh means, preferably, light-emitting semiconductor layers are laminated such that a semiconductor layer which emits light of a shorter wavelength is provided closer to a light extraction side of the light-emitting element. The term xe2x80x9clight-emitting semiconductor layerxe2x80x9d refers to the aforementioned low-energy-band-gap layer which emits light spontaneously. However, the term xe2x80x9clight-emitting semiconductor layerxe2x80x9d encompasses a semiconductor layer which emits light secondarily, inductively, or indirectly, through conversion of UV light of relatively short wavelength to visible light of long wavelength by means of, for example, photoluminescence. That is, the term xe2x80x9clight-emitting semiconductor layerxe2x80x9d encompasses the aforementioned impurity-added semiconductor layer.
According to the eleventh means, the amount of light of short wavelength converted to light of long wavelength can be reduced, and extraction efficiency of emitted light can be enhanced.
Therefore, unintended change of color of light emitted from the light-emitting element, which is attributed to conversion of the wavelength of the light, is easily avoided. According to the eleventh means, the effect of such conversion (i.e., secondary effect) on the color of light emitted from the light-emitting element is not necessarily a strong consideration, and the light-emitting element can be designed with relative ease. In addition, chromaticity of light emitted from the element can be regulated easily.
When the intensity of visible light of short wavelength (e.g., blue light) emitted from a low-energy-band-gap layer is relatively low, the eleventh means is effectively employed for maintaining the intensity at a certain level. However, the terms xe2x80x9cshort wavelengthxe2x80x9d and xe2x80x9clong wavelengthxe2x80x9d are used merely on a relative basis.
According to the first through eleventh means, the aforementioned object of the present invention can be effectively attained.